Well having a new Cybook I would say it is pretty bad at displaying pdf files for the following reasons.
1. Large pdf files cause it to crash.
2. 20% of other pdf files cause it to crash.
3. Can only fit to width/page or height. with no zoom ability
4. Doesn't use a bit of sense and remove whitespace at edges
5. In landscape mode doesn't scroll by a display page, so you need to scroll 3-5 times to get the new text at the top of the display.
Use a laptop is my recommendation.